This invention relates to communications and avionics frequency standards, frequency synthesizers, and more specifically to an integrated frequency source with an integrated frequency standard and frequency synthesizer.
Frequency standards and frequency synthesizers are widely used in radio communications, avionics, and other applications to provide reference frequencies and frequencies needed to generate desired output frequencies. Currently the frequency standard and the frequency synthesizer are designed and constructed as two separate modules.
With current practice, the frequency standard signal is distributed throughout the radio or other equipment. The frequency synthesizer and other modules in the radio must then condition the frequency standard signal to meet the module""s needs such as filtering, multiplying up in frequency, dividing down in frequency, etc. thereby requiring additional circuitry in each module. Typically, the frequency standard is a crystal oscillator with low frequency signals such as about 10 MHz due to optimum crystal design at that frequency. The frequency standard must maintain a constant output frequency with variations in temperature. The frequency standard may be a temperature compensated crystal oscillator (TCXO), an ovenized crystal oscillator, or other types of frequency standards known in the art.
The TCXO maintains a constant output frequency by pulling of the frequency standard crystal requiring the output of the frequency standard to be a voltage controlled crystal oscillator (VCXO). The VCXO requires a crystal that is easily pulled to frequency (low inductance) and circuitry to pull the crystal to frequency such as a varactor. The introduction of the circuitry to pull the crystal to frequency allows spurious frequencies to modulate the frequency standard. The requirement to pull the crystal also requires a low crystal current, which means the phase noise can degenerate. If a higher crystal current is used the phase noise of the oscillator can be lowered but pullability of the crystal is reduced. Low phase noise is required from the frequency standard to optimize synthesizer and ultimately radio performance. Multiplication of the frequency standard signal to achieve the desired output frequencies also multiplies the phase noise requiring low reference frequency phase noise. The very nature of a VCXO in a TCXO limits the achievable phase noise.
An ovenized crystal oscillator can achieve better phase noise than a TCXO since the ovenized crystal oscillator does not have the circuitry to pull the crystal to frequency. The required frequency accuracy in an ovenized crystal oscillator is obtained by placing the crystal in an oven to maintain the crystal at a constant temperature. However, the ovenized crystal oscillator is not suited for most communications and avionics applications due to its high power consumption and large size.
The development of a time compensated clock oscillator (TCCO) has enabled significant improvements in frequency stability approaching that of ovenized oscillators with reduced size and power consumption. However current TCCO frequency standard designs employ a VCXO with the attendant problems of a VCXO in a TCXO. Current TCCO frequency standards are implemented as standalone module.
Several different approaches exist for implementation of frequency synthesizers as standalone modules. Among these are indirect phase lock loop synthesizers (PLL) and direct digital synthesizers (DDS). Each of these approaches has advantages and disadvantages. A desirable feature in frequency synthesizers is small frequency step size. Indirect phase lock loop synthesizer step size is limited to the reference frequency. Indirect phase lock loop synthesizers can offer small step size with a low reference frequency but a low reference frequency results in a narrow bandwidth loop with slow tune times.
Modulated fractional divider (MFD) phase lock loop synthesizers have been developed that provide very small frequency step size with high frequency standard reference frequencies. This is possible with the addition of a modulated fractional divider in the phase lock loop of a conventional synthesizer that divides by fractional values.
A direct digital synthesizer (DDS) may be used to generate small frequency step sizes with a high reference frequency. However, the DDS output has high spurious content and may require the use of a phase lock loop to filter the DDS output.
With current practice of building the frequency standard and the frequency synthesizer as two separate modules requires that each module is packaged separately and each module contain its own controller and associated parts. Reducing the parts count in the frequency standard and synthesizer would allow a reduction in size and cost with an increase in reliability.
What is needed is an integrated frequency standard and frequency synthesizer design that eliminates the VCXO and associated circuitry to improve noise and spurious performance while eliminating a module and reducing parts count.
An integrated frequency source comprising an integrated frequency standard and an integrated frequency synthesizer for generating a desired frequency over a temperature range is disclosed. The integrated frequency standard receives frequency control data and generates synthesizer control data and a reference signal. The integrated frequency synthesizer is connected to the integrated frequency standard and generates an output signal at the desired frequency from the synthesizer control data and the reference signal.
The integrated frequency standard further comprises a reference oscillator for generating the reference signal and a temperature sensor for measuring temperature and providing a measured temperature signal. A microprocessor is connected to the reference oscillator, the temperature sensor, and the frequency synthesizer for generating the synthesizer control data from the frequency control data, the reference signal, and the measured temperature signal.
The microprocessor uses a lookup table to determine a reference frequency error at a measured temperature and then calculates the synthesizer control data for that temperature. The microprocessor calculates the synthesizer control data needed to compensate for a change in reference frequency with temperature.
The integrated frequency synthesizer of the integrated frequency source may be a modulated fractional divider phase lock loop that further comprises a modulated fractional divider that divides in accordance with the synthesizer control data to provide the desired output frequency over temperature. A direct digital synthesizer or other frequency synthesizer capable of tuning in small frequency steps may also be used.
It is an object of the present invention to provide an integrated frequency source with an integrated frequency standard and an integrated frequency synthesizer in a single module.
It is an advantage of the present invention to provide an integrated frequency source that eliminates a voltage controlled crystal oscillator to improve phase noise and spurious signal performance.
It is a feature of the present invention to allow a reduction in parts count, cost, and size.